tri.cpp

#include <stdio.h>
#include <algorithm>

// print regular triangle 
// for n = 5
// X
// XX
// XXX
// XXXX
// XXXXX
// ----
void print_reg_triangle(int n) {
  for (int fill = 1; fill <= n; fill++) {
    for (int i = 0; i < fill; i++) printf("X");
    printf("\n");
  }
}

// print upside down (usd) triangle 
// for n = 5
// XXXXX
//  XXXX
//   XXX
//    XX
//     X
// ----
void print_usd_triangle(int n) {
  for (int i = 0; i < n; i++) {
    // add leading spaces
    for (int j = 0; j < i; j++) printf(" ");
    // fill in triangle for this line
    for (int j = 0; j < n - i; j++) printf("X");
    printf("\n");
  }
}

int main() {
  int n = 5;
  void (*current_print) (int) = &print_reg_triangle;
  void (*next_print) (int) = &print_usd_triangle;

  for (int i = 0; i < n; i++) {
    current_print(n);
    printf("----\n");
    // flip flop between regular and upside down printing
    std::swap(current_print, next_print);
  }
}